A source of sensory cells and neurons for regeneration of inner ear cells would provide a valuable tool for clinical application because neurons and hair cells could be employed in cell replacement therapy for hearing loss. Recent work has shown that hair cells and neurons can be differentiated from endogenous stem cells of the inner ear (Li et al., Nat Med 9, 1293-1299 (2003); Rask-Andersen et al., Hear Res 203, 180-191 (2005)) and other work has shown that endogenous cells of the sensory epithelium can be converted to hair cells when the proneural transcription factor, Atoh1, is expressed exogenously (Izumikawa et al., Nat Med 11, 271-276 (2005); Zheng and Gao, Nat Neurosci 3, 580-586 (2000)) and yet the endogenous stem cells of the inner ear do not spontaneously generate hair cells. Injection of whole bone marrow to reconstitute a lethally irradiated mouse resulted in engraftment of these cells in areas occupied by inner ear mesenchymal cells and fibrocytes but did not yield hair cells (Lang et al., J Comp Neurol 496, 187-201 (2006)).